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Before posting, please read https://www.cgsecurity.org/testdisk.pdf

I have a GPT disk that had a single NTFS partition on it (other than the MSR partition, that is). A rogue boot manager overwrite the GPT header and partition tables. Fortunately, GPT and NTFS both keep backup copies of themselves elsewhere on the disk, and I was able to find and restore these with testdisk. However, the partition now shows up as "MS Data" type. When I use testdisk to try to change the type, this is what happens:

1. Highlight the partition in question.
2. Press "T" to change type
3. A screen comes up with many different partition types, MS Data is pre-selected.
4. Press enter to proceed.
5. A second screen with more partition types comes up. Select NTFS. Press Enter.
6. Partition still shows up as MS Data, not NTFS.

There does not seem to be any way to advance from the first partition type selection screen to the second, except by highlighting a choice and pressing Enter. Whatever is then selected on the second page (e.g., NTFS) is subsequently ignored when Enter is pressed again.

Did you use the menu Advanced and Type?
Because it looks like that your partition is available in your GUID-partition table?
MS Data is the correct one.
After adding NTFS to your partition using Type in the menu Advanced you will be able to confirm at Boot and in case to Rebuild your boot sector and you can check List to have a try to list your data.
Please let me know.
In case your files are listed, you'd be able to write your boot sector using the menu Write.

Advanced menu and Type did it. Rebuilt the boot sector, and was able to list files. Then I used good old chkdsk to rebuild the NTFS file tables, and made a 100% recovery. :)

All of this was caused by a non-GPT aware boot loader (GAG) that overwrote the GPT header and protective MBR. The thing is, GAG installed itself on the GPT drive when it had no business doing so. The GPT drive is used only for data, not booting.

So now I am looking for a GPT-aware boot loader that won't corrupt the GPT disk again, so I can multi-boot Win Vista, 7, and Ubuntu. Any other OSes I want to use, I'll run in VMs. :)

In the meantime, I'd like to make a backup of my GPT disk partition tables, in case this ever happens again. I'm not sure if I should use testdisk or gdisk for that purpose.

I think GRUB is able to boot on GPT, respecting the protective MBR. Don't use GAG, BURG or old bootloaders.
Your GPT header & table are already saved at the end of your disk, backward. In case of crash, gdisk is able to repair.
If you want to duplicate those (good Idea because sometimes gdisk isn't easy to use for repairing GPT) use a bit/bit cloning tool like dd or gnu-ddrescue. Remember that your GPT may have 128 entries...

You'll probably found good help in ubuntu doc or forums with multi-boot.